Interactives . The Rock Cycle . Types of Rocks Types of Rocks Rocks are not all the same! The three main types, or classes, of rock are sedimentary, metamorphic, and igneous and the differences among them have to do with how they are formed. Sedimentary Sedimentary rocks are formed from particles of sand, shells, pebbles, and other fragments of material. Together, all these particles are called sediment. Gradually, the sediment accumulates in layers and over a long period of time hardens into rock. Generally, sedimentary rock is fairly soft and may break apart or crumble easily. You can often see sand, pebbles, or stones in the rock, and it is usually the only type that contains fossils. Examples of this rock type include conglomerate and limestone. Metamorphic Metamorphic rocks are formed under the surface of the earth from the metamorphosis (change) that occurs due to intense heat and pressure (squeezing). The rocks that result from these processes often have ribbonlike layers and may have shiny crystals, formed by minerals growing slowly over time, on their surface. Examples of this rock type include gneiss and marble.

Igneous Igneous rocks are formed when magma (molten rock deep within the earth) cools and hardens. Sometimes the magma cools inside the earth, and other times it erupts onto the surface from volcanoes (in this case, it is called lava). When lava cools very quickly, no crystals form and the rock looks shiny and glasslike. Sometimes gas bubbles are trapped in the rock during the cooling process, leaving tiny holes and spaces in the rock. Examples of this rock type include basalt and obsidian. Here's a chart of some of the key characteristics that can help you identify the rocks within these three main classes. Crystals Small, flat surfaces that are shiny or sparkly, like tiny mirrors. Fossils Imprints of leaves, shells, insects, or other items in the rock. Gas bubbles "Holes," like Swiss cheese, in the rock. Glassy surface A shiny and smooth surface, like colored glass. Ribbonlike layers Straight or wavy stripes of different colors in the rock. Sand or pebbles Types of volcanic rock | Sciencelearn Hub Types of volcanic rock Rocks are not all the same. Some are heavy, some are light. Others are dark, while some can be almost pure white.

Even igneous rocks that are all formed from magma in the Earth’s mantle can look very different. Igneous rocks Rocks are broadly classified into three groups – igneous, sedimentary and metamorphic . Igneous rocks are formed from magma in the Earth’s mantle. They generally don’t contain fossils, don’t react with acids, don’t usually contain obvious layers, can be made of different minerals, sometimes have holes or bubbles and may be glassy in appearance. Volcanologists look for these igneous rocks so that they can learn more about where these rocks have come from and whether they were formed during a volcanic eruption. Geologists use the visual appearance of the rock as an initial clue to its composition but will then verify their ideas using specialised techniques. For example, scientists at The University of Auckland use an electron microprobe to measure the exact quantities of silica , iron , magnesium and many other chemicals that are in rock samples they collect. This information helps them to classify the rock and may give them direct clues about the volcano and the eruption that formed the rock. Lava solidifies to rock New Zealand has 3 main types of volcanoes, and each has been formed from a different type of magma.

Once the lava has erupted, it cools and solidifies into rock: Basalt magma often forms shield volcanoes. Andesite magma often forms cone volcanoes. Rhyolite magma often forms calderas . Depending on how much gas the magma contains, it can also form cone volcanoes. Basalt Image: Columnar basalt The Earth’s crust is mainly basalt rock. It is a heavy, dark, grainy rock. Basalt is associated with great rock columns that are found in many places around the Earth, for example, the Organ Pipes in Dunedin or the Giant’s Causeway in Ireland. Basalt magma is formed at high temperatures (around 1,200ºC). When it comes out of the volcano, it is hot and liquid. It contains very little silica (less than 50%) and a lot of magnesium and iron, which makes the rock look dark. The Auckland volcanic field has erupted this type of hot, runny iron-rich lava, and the landscape is dotted with mountains made from basalt and scoria (a red-coloured rock that contains large amounts of iron-rich minerals). Both rock types are excavated for building materials and landscaping.

Andesite Image: Andesite Andesites are lighter coloured than basalt because they contain less iron and more silica (50–60%). Some scoria rocks fall within the andesite classification because of their chemical composition. Magma that contains andesite is generally around 800–1,000ºC and forms steep-sided cone volcanoes (stratovolcanoes). Mount Ngāuruhoe is an example of an andesite volcano. Rhyolite Image: Pumice Rhyolite is light-coloured or white – this is a clue that the rock contains a lot of silica (more than 70%) and not much iron or magnesium. Rhyolitic magmas are associated with low temperatures (750–850ºC) and are often thick, which means gases can’t escape. Some rhyolitic rocks are quite light, for example, pumice , which may still have evidence of the bubbles of gas trapped as the rock solidified. Nature of Science Classification helps scientists organise things into groups. In rock classification, such grouping can help geologists see patterns and perhaps explain the reasons for rocks looking similar.

Metadata Rock Types Rock Types About Rock types Rock type is usually defined as a particular kind of rock having a specific set of characteristics 1 . Rock types are specific assemblages of minerals (most rocks are composed of minerals). Rocks are much more vaguely defined than minerals . Even rocks within one rock type may have a highly variable composition. Some important igneous rocks from left to right: gabbro, andesite, pegmatite, basalt, pumice, porphyry, obsidian, granite, and tuff. Still, there is a fundamentally sound reason to talk about such a vague concept as a rock type because these assemblages occur again and again in many different locations. Rock type is like a biome (desert, savannah, rain forest). One particular desert may greatly differ from another (one is sandy and another rocky), but they both share something that is common (low precipitation). Exactly the same is true with rock types — one granite may be white and another red, but they both have a similar composition (major minerals are feldspar and quartz). Here is a list of major rock types recognized by geologists. Immense number of rock types have been defined (their definitions often overlapping) and many rock classifications exist.

However, there seems to be a fair number of terms and classification principles that have successfully stood the test of time. Contents Igneous rocks Amphibolite · Anthracite · Blueschist · Charnockite · Eclogite · Epidosite · Fenite · Gneiss · Granulite · Greenschist · Greisen · Hornfels · Marble · Metapelite · Migmatite · Mylonite · Phyllite · Quartzite · Schist · Serpentinite · Shungite · Skarn · Slate · Soapstone · Suevite Igneous rocks It all starts with igneous rocks . The Earth is believed to have been entirely molten in its early stages of development. Hence, all other rock types are derivatives of igneous rocks. Andesite is a volcanic rock with a so-called average composition. It is more felsic than basalt and more mafic than dacite. The main minerals are plagioclase and hornblende. Andesite is a common rock of subduction zone volcanoes. It is generally dark gray and may contain phenocrysts of plagioclase (usually andesine), biotite, and hornblende.

Andesite is an extrusive equivalent of diorite. ^ Anorthosite is a plutonic rock which is composed of plagioclase (>90%). Anorthosite is a cumulate rock. Its constituent plagioclase crystals floated to the top of the mafic magma chamber and crystallized there as almost monomineralic plagioclase rock. Some anorthosites display very beautiful display of colors known as labradorescence and are therefore valuable ornamental stones. Anorthosite is a common rock on the Moon. The highlands of the Moon which we see as brighter areas are largely composed of this rock type. ^ Aplite is a fine-grained and usually light-colored plutonic rock. Aplite usually solidified in a narrow crack where heat loss to the adjacent rocks was rapid and crystals did not have much time to grow in size. Aplites may have a variable composition from granite to gabbro, but the term “aplite” is mostly used to describe fine-grained granitic rocks.

^ Basalt is a very common dark-colored volcanic rock composed of calcic plagioclase (usually labradorite), clinopyroxene (augite) and iron ore (titaniferous magnetite). Basalt may also contain olivine, quartz, hornblende, nepheline, orthopyroxene, etc. Basalt is a volcanic equivalent of gabbro. Oceanic crust is mostly composed of basalt or rocks with the same composition (gabbro, diabase). ^ Basanite is a volcanic rock. It is similar to basalt but contains less silica and usually more alkali metals. Basanite may contain olivine, pyroxene, and feldspathoid phenocrysts. Similar rock without olivine is tephrite. Limburgite is a glassy nepheline basanite. ^ Carbonatite is an igneous rock that is predominantly composed of carbonate minerals. Both volcanic and plutonic carbonatites are known. Alvikite is a volcanic calcite carbonatite, sövite is a plutonic calcite carbonatite. Carbonatite is generally associated with alkaline igneous rocks.

Carbonatite is a host rock of several important commodities, most notably apatite, rare earth elements and niobium. ^ Dacite is a felsic volcanic rock intermediate in composition between andesite and rhyolite. Dacite is a common rock of subduction-related volcanoes on the active continental margins (Cascades in the northwestern USA, for example). Dacitic magma in the crust solidifies as intrusive rock granodiorite. ^ Diabase is a dark-colored igneous rock. It is compositionally equivalent to gabbro and basalt but texturally between them. Diabase is a common rock type. It occurs mostly in shallow intrusions (dikes and sills) of basaltic composition. Diabase grades to basalt when it solidifies rapidly and to gabbro when more time is given to the crystals to grow. ^ Diorite is a plutonic rock with sodium-rich plagioclase and hornblende. Diorite is compositionally equivalent to volcanic rock andesite. It is intermediate in composition between felsic granite and mafic gabbro.

^ Dunite is an ultramafic plutonic rock that is composed almost exclusively of olivine (>90%). Dunite is a sub-type of peridotite. These rocks are abundant in the Earth’s mantle but occur relatively rarely in continental rocks. Olivine rock is a synonym of dunite. Olivinite is a variety that contains more magnetite than chromite. ^ Foyaite is a rare igneous rock consisting of nepheline and alkali feldspar with a trachytic texture. Foyaite is a variety of nepheline syenite. ^ Gabbro is a coarse-grained and usually dark-colored igneous rock. Gabbro is an intrusive rock. Igneous rocks with a similar composition are basalt (extrusive equivalent of gabbro) and diabase (the same rock type could be named dolerite or microgabbro instead). ^ Granite is a very common plutonic rock with feldspar and quartz as the main constituents. Granite is an igneous rock, but some high-grade metamorphic rocks are compositionally very close and have been sometimes referred to as granites also.

^ Granodiorite is a granitoid (quartz-rich plutonic rock) in which the dominant feldspar is plagioclase. Granodiorite is compositionally between other granitoids granite and tonalite. Granodiorite contains more quartz than diorite. This rock type is frequently misidentified as granite. ^ Harzburgite is what remains when basalt is taken out of the mantle. Harzburgite is a sub-type of peridotite. Harzburgite is composed of orthopyroxene and olivine, it is one of the ultramafic rocks and belongs into the peridotite group. These are rocks that are abundant in the mantle but scarce on the surface. ^ Hawaiite is a volcanic rock similar to basalt but contains more alkali metals. It is a sodic variety of trachybasalt. ^ Hornblendite is an ultramafic plutonic igneous rock that is composed almost entirely of hornblende. Hornblendite crystallized from water fluid-rich magma which favored the formation of hydrous hornblende over anhydrous pyroxene. ^ Ignimbrite is a fiery rock.

It is a pyroclastic rock formed by very hot ground-hugging cloud of volcanic ash, blocks, and gases known as pyroclastic flow or pyroclastic density current. Ignimbrite is synonymous with flood tuff, welded tuff, ash-flow tuff and pyroclastic flow deposit 3 . ^ Ijolite is a rare igneous rock consisting of nepheline and pyroxene. Ijolite is a variety of foidolite (feldspathoid-rich plutonic rock in which fledspathoids are dominating over feldspars). ^ Kimberlite is an ultramafic rock consisting of olivine and one or more of the following: phlogopite, serpentine, diopside, monticellite, calcite. Kimberlite contains no leucite. Kimberlite is an economically important rock type because it is the principal source (host rock) of diamonds. Kimberlite occurs in pipe-like rock body called diatreme, which formed through violent gas-charged volcanic eruption. ^ Komatiite is an ultramafic lava. Komatiite is compositionally similar to intrusive rock peridotite.

Some volcanic rocks (picrites) may be ultramafic because they are olivine cumulates, but komatiite is a true ultramafic lava that formed as a result of very extensive partial melting. Komatiites do not form anymore. Most likely because the Earth has cooled down and does not allow such a high degree of partial melting to take place. Komatiite lavas commonly demonstrate spinifex texture — long olivine crystals in the upper part of a lava flow. ^ Limburgite is a variety of basanite with a glassy groundmass and nepheline as the main feldspathoid mineral. Limburgite contains no feldspars. ^ Monzonite is a plutonic rock intermediate in composition between syenite and diorite. Monzonite contains approximately equal amount of K-feldspar and plagioclase. Monzonite contains no or small amount of quartz and the content of alkali feldspar and plagioclase are roughly equal. ^ Nepheline syenite is a plutonic rock rich in alkali metals and relatively poor in silica.

As a result of that it contains alkali minerals like feldspathoid nepheline in addition to alkali feldspar, alkali amphibole and/or alkali pyroxene. Quartz is absent. Phonolite is a volcanic analogue of nepheline syenite. ^ Norite is a gabbroid rock in which the pyroxene mineral is orthopyroxene. ^ Obsidian is a volcanic glass — a rock type that in pure form contains no crystals. Obsidian is usually rhyolitic in composition and black or reddish in color. Obsidian is frequently associated with pumice. ^ Pegmatite is an exceptionally coarse-grained plutonic rock. Most pegmatites have a granitic composition. Simple pegmatites contain common minerals like feldspar, mica and quartz. Complex pegmatites are rich in unusual chemical elements and minerals which makes pegmatite a potential mineral resource. ^ Peridotite is the principal rock type in the Mantle. Unaltered peridotite is usually dark green in color but most peridotites we see in the surface are altered to a metamorphic rock type serpentinite.

There are several sub-types of peridotite: harzburgite, dunite, lherzolite, etc. ^ Phonolite is an alkaline volcanic rock that is composed of fine-grained alkali feldspar, nepheline, alkali amphiboles and alkali pyroxenes. Phonolite may be porphyritic. It is an extrusive equivalent of plutonic rock nepheline syenite. ^ Picrite is a mafic igneous rock similar to basalt but with more olivine. Picrite may form as an olivine cumulate rock. Olivine crystals may settle to the lower part of a lava flow, forming an ultramafic rock. ^ Porphyry is an igneous rock characterized by porphyritic texture. Porphyritic texture is a very common texture in igneous rocks in which larger crystals (phenocrysts) are embedded in a fine-grained groundmass. Porphyritic texture indicates that the crystallization process started well before the lava was extruded to the surface. ^ Pumice is a lava froth — an extremely vesicular and light-weight volcanic rock with a variable composition. Pumice is unusually light-weight for a rock — it floats in water.

Pumice is usually light-colored and has a felsic composition. Pumice forms during explosive volcanic eruptions. ^ Pyroxenite is a plutonic rock that is composed almost exclusively (over 90%) of pyroxene minerals. Pyroxenite is a cumulate rock usually associated with other ultramafic (peridotite) and mafic (gabbro) rocks. ^ Rapakivi is a variety of granite with K-feldspar phenocrysts that are mantled with plagioclase. Rapakivi is an anorogenic rock occurring in intra-plate settings. ^ Rhomb-porphyry is a rare porphyritic rock with abundant wedge-shaped anorthoclase phenocrysts. Rhomb-porphyry from Norway is a volcanic equivalent of larvikite. Rhomb-porphyry is compositionally trachyandesite (latite). ^ Rhyolite is a felsic volcanic rock. It is an extrusive analogue of granite. Rhyolite is associated with explosive volcanoes. Rhyolitic rock, because of high viscosity, may occur as pumice and obsidian.

Porphyritic rocks are also frequently rhyolitic in composition. ^ Scoria is a highly vesicular mafic dark-colored volcanic rock. It is similar to pumice but darker in color, not as light-weight, and more mafic in composition. Vesicules in scoria are usually larger than in pumice. ^ Sövite is a type of plutonic carbonatite where the dominant carbonate mineral is calcite. Sövite is the most common variety of carbonatite. It is often mined as a host rock of various mineral resources. ^ Syenite is a plutonic rock that contains little or no quartz and in which K-feldspar dominates over plagioclase. Syenite is like a granite but without quartz. In nepheline syenite some of the alkali feldspar is replaced by nepheline and quartz is completely absent. ^ Tephrite is a volcanic rock resembling basalt, but it is rich in feldspathoid minerals. This makes it very similar to basanite, but tephrite contains very little olivine.

^ Tonalite is a granitoid in which almost all the feldspar (>90%) is plagioclase. Trondhjemite and plagiogranite are varieties of tonalite. ^ Trachyandesite is a volcanic rock intermediate in composition between trachyte and andesite. Trachyandesite is compositionally close to intrusive rock monzonite. Rhomb-porphyry is a variety of trachyandesite. ^ Trachyte is a feldspar-rich rock. It is a volcanic equivalent of syenite. Trachyte grades into rhyolite with increasing quartz content. ^ Troctolite is a variety of gabbro without pyroxene. Troctolite is a cumulate rock associated with other gabbroic rocks like gabbro, norite, and anorthosite. ^ Trondhjemite is a leucocratic variety of tonalite composed of sodic plagioclase, quartz and biotite. ^ Tuff is a volcanic analogue of sandstone. It is a consolidated and lithified volcanic ash.

^ Sedimentary rocks Sedimentary rocks are products of the consolidation of loose sediments which are in most part bits and pieces of other disintegrated rocks. There are three main types of sedimentary rocks: mudstone, sandstone, limestone. Sure, this is very simplistic approach, but volumetrically almost all sedimentary rocks fall under these categories if we give them relatively broad definition e.g. sandstone also includes siltstones and conglomerates, and limestone includes all sedimentary carbonate rocks. There are notable exception that do not fit into this classification scheme like evaporites, coal, and chert, but volumetrically they are clearly less important. In the list below, of course, these exceptions are not neglected. Arkose is a feldspar-rich (25%) sandstone. Arkose is usually coarse-grained and composed of angular grains. These properties are characteristic of relatively immature sediments. Arkose is usually a disintegration product of nearby granitic rocks. ^ Banded iron formation is a chemogenic metasedimentary rock consisting of alternating silica- and iron-rich bands. Banded iron formation (BIF) is the principal source of iron. It is perhaps the most important rock type as our society is heavily reliant on iron extracted from it.

^ Bauxite is an aluminum-rich sedimentary rock. It is a principal ore of aluminum. Aluminum in bauxite is hosted by aluminum hydroxide minerals, mostly gibbsite. The major impurities are iron oxides and hydroxides (which give reddish color to most bauxites) and clay minerals. Bauxite is a weathering product of aluminum-bearing rocks. ^ Bog iron is a type of iron ore composed of limonite with impurities like clay or plant debris. Bog iron is porous rock formed in bogs, wetlands, and shallow lakes. Iron is precipitated from water by the oxidizing action of bacteria and algae. Bog iron is common in cold temperate zones of Scandinavia and North America. ^ Breccia is a coarse-grained shattered rock that is composed of highly angular fragments held together by a mineral cement. Breccia may be a result of several different geological processes: talus formation, explosive volcanic eruptions, tectonic movements, rock avalanches, etc. ^ Chalk is a soft, friable and porous variety of limestone. Chalk is composed of calcareous tests of marine microorganisms like coccoliths and foraminifers.

The best known chalks are of Cretaceous age. Chalk is often associated with chert (flint). ^ Chert is a hard and compact sedimentary rock, consisting dominantly of very small quartz crystals. Chert occurs mostly in carbonate sedimentary rocks as nodules or layers. Flint is a dark-colored variety of chert. ^ Coal is a combustible rock containing more than 50% by weight of carbonaceous material. Coal in most cases is a lithified peat. Coal is a major fossil fuel, although in recent years its importance in the developed world has been declining. ^ Conglomerate is a lithified gravel (rounded rock fragments or mineral grains larger than 2 mm in size). Conglomerate contains smaller grains (matrix) in the interstices. Conglomerate is a former river bed, cobble beach or another environment where the constituent clasts were rounded. Breccia is somewhat similar rock with angular clasts. ^ Coquina is a detrital limestone consisting of shells or shell fragments. Coquina is a coarse-grained (particles over 2 mm in size) conglomeratic rock. Finer carbonate rock made of fossil fragments is calcarenite.

^ Diamictite is a poorly sorted or non-sorted terrigenous non-calcareous sedimentary rock. Diamictite is composed of widely variable clast sizes from clay to boulders. Diamictite is a nongenetic term (purely descriptive, no formation mechanism is implied). Most diamictites are glacial in origin (tillite), but some are volcanic (lithified lahars), submarine (turbidites), mass wasting breccias, etc. ^ Diatomite (diatomaceous earth) is a light-colored sedimentary rock composed chiefly of siliceous shells (frustules) of diatoms. Diatomite is soft and friable rock. Most diatomite deposits are marine but some were formed in lakes. Diatomite has several industrial uses. ^ Dolomite (dolostone) is a carbonate rock containing more than 50% by weight of the mineral dolomite. Dolomite may be a reservoir rock for hydrocarbons (oil & gas). ^ Evaporite is any rock type formed as a result of consolidation of salts crystallized from hypersaline water.

Evaporites may form in lagoons where evaporation is intensive due to hot climate and the influx of new lower salinity water is (periodically) restricted, but in many cases evaporites formed by replacing the original minerals of sedimentary rocks with the new ones from brines percolating the rocks. Rock salt and gypsum (as a rock type) are examples of evaporites. ^ Flint is basically a synonym of chert. The term is especially often used for a homogenous dark gray or black variety of chert. Flint is more widely used name for the rock in general public, but the term “chert” is usually preferred in geological literature (regardless of the color of the rock). ^ Graywacke is an old and vaguely defined rock name for a coarse-grained firmly indurated sandstone that is dark-colored and contains lots of clay and lithic fragments. Graywacke is poorly sorted marine rock. It is nowadays interpreted to be of turbiditic origin.

This rock name has been extensively criticized since 1839 when Roderick Murchison said that “it has already been amply shown that this word should cease to be used in geological nomenclature, and…is mineralogically worthless”. Almost two centuries later many geologists seem to share the view of Murchison. But regardless of that, the rock name continues to be used at least informally and as a field term for a dark-grained “dirty” sandstone about which “you can’t tell much about in the field”. ^ Gypsum as a rock type is an aggregate of gypsum crystals. Gypsum is an evaporite and is usually associated with other evaporite minerals like halite and anhydrite. Gypsum occurs in different forms: massive alabaster, fibrous satin spar, selenite crystals. Gypsum is an important industrial mineral. It is used mostly in the construction industry. ^ Laterite is a reddish weathering product of basaltic (mostly) rocks. Laterite forms in hot to temperate climate and is a residual product of weathering. ^ Lignite is a variety of low-quality coal that is intermediate in coalification between coal and sub-bituminous coal.

Brown coal is a synonym of lignite. ^ Limestone is a sedimentary rock that is chiefly composed of calcium carbonate. Limestone is a major rock type. It is mostly marine and biogenic, but can also form as a chemical precipitate (travertine, tufa). Oolite, chalk, calcarenite, coquina, etc. are varieties of limestone. ^ Oil shale is a rock that yields oil (shale oil) upon heating without oxygen. Organic (burning) matter in oli shale is kerogen. Oil shale is a variety or shale. Oil shale reserves are abundant, but it is seldom used as a fossil fuel because it is usually low in calorific value and contains lots of impurities. ^ Oolite is a rock made up of small, generally rounded grains with internal growth bands that are named ooids. Most oolites are limestones (ooids are composed of calcium carbonate). ^ Phosphorite is a sedimentary rock containing significant amount of phosphate material. Phosphorite is a rock containing enough phosphate to be of economic interest. Phosphatic matter may be in ooids, pellets, nodules, shells, bones, etc.

^ Sandstone is a lithified sand. There are many subtypes of sandstone: arkose, orthoquartzite, grit, flagstone, graywacke, and many others. Sandstone is among the most widespread sedimentary rocks. The most abundant constituent mineral in most sandstones is quartz. ^ Shale is a laminated rock consisting of mostly clay minerals. Shale is a lithified mud. Shale and similar mudstones together make up more than half of all the sedimentary rocks. Shale is a source rock of crude oil and natural gas. ^ Siltstone is an intermediate member between sandstone and shale in both grain size and properties. Loess is a variety of wind-deposited siltstone. Pure siltstone is composed of very fine dusty grains, but it may contain significant amount of sand and clay. ^ Tillite is a poorly sorted glacial sedimentary rock — consolidated glacial till. Tillite in many cases is referred to as diamictite, although the latter is purely descriptive term and may have several formation mechanisms. ^ Travertine is a chemically precipitated limestone. Stalactites and other speleothems in karst caves are made of this rock type.

Travertine is also filling calcitic veins. Tufa is a soft and porous chemically precipitated calcitic rock. ^ Turbidite is not a rock type in a strict sense. It is a sedimentary complex consisting of alternating layers of muddy and silty sediments. Turbidite is inferred to have been deposited underwater by a turbidity current. Flysch is a frequently encountered term in older geological literature that means the same thing. ^ Umber is a marine sediment associated with black smokers. Umber is brown in color (brown earth, darker than ocher) and valued as a natural brown pigment. Umber occurs in Cyprus-type massive sulfide deposits. ^ Metamorphic rocks Metamorphic rocks are derived from other pre-existing rocks by mineralogical and/or structural changes. Metamorphism takes place in the solid state and at elevated pressure and temperature generally at depth in the crust 1 . Amphibolite is a metamorphic rock composed of amphiboles (usually hornblende) and plagioclase. Amphibolite should not be confused with the metamorphic facies of the same name. Amphibolite is a widespread rock type.

^ Anthracite is a type of coal with a highest rank. Coal rank measures the carbon content which is in correlation with calorific value and metamorphic grade. Anthracite has a very high carbon (over 90%) and low volatile content (below 5%). ^ Blueschist is a bluish metamorphic rock containing amphibole glaucophane. Blueschist forms at high pressure and low temperature (subducting slab of lithosphere). Its protoliths are basaltic rocks. ^ Charnockite is an orthopyroxene-bearing anhydrous metamorphic rock. Charnockite is often referred to as granite. This practice is in most cases misleading because charnockitic rocks did not crystallize from magma. ^ Eclogite is a beautiful rock consisting of bright green pyroxene omphacite and red garnet. Eclogite forms at high-pressure conditions. Its protolith is a basaltic rock. ^ Epidosite is a hydrothermally altered basaltic rock consisting of epidote and quartz. Epidosite is a product of hydrothermal metamorphism (alteration). Its parent rock was a mafic igneous rock (basalt, diabase).

^ Fenite is a metasomatic rock surrounding complexes of carbonatite-alkaline rocks. Fenite is an altered former granitic (or similar) rock. Fenitization is a process which replaces some or all of the original minerals by new and generally more alkaline phases (alkali pyroxene, alkali amphibole, feldspathoids, etc.). ^ Gneiss is a banded coarse-grained metamorphic rock that formed deep in the crust beneath forming mountain ranges. The chemical composition may be highly variable, although most gneissose rocks are granitic. ^ Granulite is a confusing term in metamorphic petrology. Usually it is a metamorphic rock of granulite facies. This is unfortunate use because metamorphic rocks should never be confused with facies names. There is a similar case with amphibolite and amphibolite facies, but amphibolite is a well-defined rock type. We know that amphibolite does not necessarily equilibrate at amphibolite facies conditions and neither are all amphibolite facies rocks amphibolites. The situation, unfortunately, is more complicated with granulites because there are no compositional defining driteria.

Therefore, rocks with widely varying compositions are named so which complicates the matter to the point of utter confusion. It has been proposed that the term should be abandoned and granofels used instead which is free of unwanted associations with metamorphic facies. Nevertheless, the term “granulite” continues to be used in the scientific literature. ^ Greenschist is a schistose metamorphic rock. Green color is given to this rock usually by chlorite, epidote, actinolite, etc. Greenschist is usually metamorphosed mafic igneous rock (metabasite). If the green mineral is chlorite, the rock should be named chlorite schist. ^ Greisen is a pneumatolytically altered granitic rock. Greisen contains lots of quartz and mica plus several exotic minerals like cassiterite, tourmaline, topaz, etc. Greised is a metamorphic rock that metamorphosed itself — hot fluids that altered the rock were provided by the cooling magma body itself. ^ Hornfels is a fine-grained compact rock rock formed by contact metamorphism (low pressure, high temperature).

^ Marble is a metamorphosed (recrystallized) carbonate rock (limestone, dolomite rock). Marble is mostly composed of calcium carbonate. Marble in geology has somewhat more restrictive meaning than in commerce. ^ Metapelite is a metamorphosed claystone (mudstone, shale). Usually either schist or slate is named so although any metamorphosed rock can be metapelite if it has a clayey protolith. These rocks are rich in aluminum and contain various Al-rich silicate phases like micas, garnet, staurolite, kyanite, cordierite, etc. ^ Migmatite is an inhomogenous rock consisting of dark-colored metamorphic rock and light-colored igneous leucosomes. Igneous or igneous-looking component of the rock is either the result or partial melting or material intruded into the rock. ^ Mylonite is a highly deformed rock that is composed of stretched and flattened mineral grains. Mylonites form deep in the crust where temperature is high enough for ductile deformations in shear zones. ^ Phyllite is a metamorphic rock (usually metapelite) intermediate in grade between slate and schist.

Minute platy crystals of sericite, chlorite, or graphite give a characteristic sheen to the cleavage surfaces which distinguishes it from duller slate. Constituent minerals in schist are coarser. ^ Quartzite is a metamorphic rock consisting largely or entirely of quartz 2 . In vast majority of cases, quartzite is a metamorphosed sandstone. ^ Schist is a strongly foliated rock rich in platy or elongated minerals. Schist forms during regional metamorphism. Composition may be highly variable. ^ Serpentinite is a metamorphosed ultramafic rock. It is composed of serpentine minerals. Serpentinite forms at the expense of olivine- and pyroxene-bearing rocks (peridotite, pyroxenite, dunite). Chlorite, talc and magnesite frequently occur in these rocks. ^ Shungite is a metamorphosed oil shale consisting of structureless carbon. Shungite is a very rare rock type and the details of its genesis are unclear. ^ Skarn is a metasomatic rock composed of calc-silicate minerals. Skarn is formed at the contact zone between carbonate rocks and silicic magma.

Skarn is often rich in hydrothermal ore minerals. ^ Slate is a fine-grained rock that can be split into slabs (slaty cleavage). Slate in the majority of cases is a compacted and metamorphosed mudstone or shale. Slate is a low- to medium-grade metamorphic rock that will pass on to phyllite, schist and gneiss with increasing burial depth. ^ Soapstone is a soft rock composed of talc with other silicates like micas, pyroxene, amphibole, chlorite, etc. Soapstone is named so because of slippery feel when touched. Soapstone may be schistose (talc schist) or massive. ^ Suevite is a shock-metamorphosed rock. Suevite is an impact breccia composed of impact melt and lithic fragments. Impact metamorphism is caused by a collision between Earth and large extraterrestrial body which is usually meteorite or asteroid. ^ References 1. Jackson, J. A. (1997). Glossary of Geology , 4th Edition. American Geological Institute. 2. Barth, T. F. W. (2007). Quartzite.

In: McGraw Hill Encyclopedia of Science & Technology , 10th Edition. McGraw-Hill. Volume 14. 722. 3. Tilling, Robert I. (2007). Ignimbrite. In: McGraw Hill Encyclopedia of Science & Technology , 10th Edition. McGraw-Hill. Volume 9. 20-21. The 3 basic rock types Ask GeoMan... What are the 3 basic types of rocks? Just as any person can be put into one of two main categories of human being, all rocks can be put into one of three fundamentally different types of rocks. They are as follows: Igneous Rocks Igneous rocks are crystalline solids which form directly from the cooling of magma. This is an exothermic process (it loses heat) and involves a phase change from the liquid to the solid state. The earth is made of igneous rock - at least at the surface where our planet is exposed to the coldness of space. Igneous rocks are given names based upon two things: composition (what they are made of) and texture (how big the crystals are). Click here for more on igneous rock composition and texture.

Click here for more on elements and minerals common in igneous rocks. Click here for more on magma and igneous rocks. Click here for more on plate tectonics and the formation of magma. Click here for a chart summarizing the main divisions of igneous rocks. Click here for more on basalt and granite. Sedimentary Rocks In most places on the surface, the igneous rocks which make up the majority of the crust are covered by a thin veneer of loose sediment, and the rock which is made as layers of this debris get compacted and cemented together. Sedimentary rocks are called secondary, because they are often the result of the accumulation of small pieces broken off of pre-existing rocks. There are three main types of sedimentary rocks: Clastic: your basic sedimentary rock. Clastic sedimentary rocks are accumulations of clasts: little pieces of broken up rock which have piled up and been "lithified" by compaction and cementation. Chemical: many of these form when standing water evaporates, leaving dissolved minerals behind. These are very common in arid lands, where seasonal "playa lakes" occur in closed depressions.

Thick deposits of salt and gypsum can form due to repeated flooding and evaporation over long periods of time . Organic: any accumulation of sedimentary debris caused by organic processes. Many animals use calcium for shells, bones, and teeth. These bits of calcium can pile up on the seafloor and accumulate into a thick enough layer to form an "organic" sedimentary rock. Click here for more on sedimentary processes and rocks (RCC). Click here for more on sedimentary rocks (GPHS). Metamorphic Rocks The metamorphics get their name from "meta" (change) and "morph" (form). Any rock can become a metamorphic rock. All that is required is for the rock to be moved into an environment in which the minerals which make up the rock become unstable and out of equilibrium with the new environmental conditions. In most cases, this involves burial which leads to a rise in temperature and pressure. The metamorphic changes in the minerals always move in a direction designed to restore equilibrium. Common metamorphic rocks include slate, schist, gneiss, and marble. Click here for more on metamorphic processes and rocks (RCC). Click here for more on metamorphic rocks (GPHS).

Types of Igneous Rocks | Types of Igneous Rocks | Types of Igneous Rocks An igneous rock is formed by the cooling and crystallization of molten rock. The term igneous is derived from ignius, the Latin word for fire. Scientists have divided igneous rocks into two broad categories based on where the molten rock solidified. Volcanic rocks (also called extrusive igneous rocks) include all the products resulting from eruptions of lava (flows and fragmented debris called pyroclasts). Plutonic rocks (also called intrusive igneous rocks) are those that have solidified below ground; plutonic comes from Pluto, the Greek god of the underworld. The initial distinction between volcanic and plutonic rocks is made on the basis of texture (fine-grained volcanic vs. coarse-grained plutonic). Volcanic and plutonic rocks are divided further on the basis of chemistry and mineral composition. The classification scheme below is based on chemistry, and is perhaps the simplest method; there are many other classification methods for igneous rocks. These rock types all have different characteristics, including temperature when fluid, viscosity (resistance to flow), composition, explosiveness, and types, amounts, and sizes of minerals.

Major Chemical Elements Forming Igneous Rocks Illustration by J. Johnson Volcanic rocks are typically divided into four basic types according to the amount of silica (SiO 2 ) in the rock (see figure at bottom, Components of Igneous Rocks, for the plutonic equivalents of these rock types): basalt consists of about 48-52% silica andesite consists of about 52-63% silica dacite consists of about 63-68% slica rhyolite consists of more than 68% silica Other major elements in varying proportion include titanium (TiO2), aluminum (A2O3), iron (FeO or Fe2O3), manganese (MnO), magnesium (MgO), calcium (CaO), sodium (Na2O), potassium (K2O, and phosphorous (P2O5). The bar graph shows the average concentration of each major element for the four basic types of volcanic rock. Classification and Flow Characteristics of Volcanic Rocks Illustration by J. Johnson The behavior of a lava flow depends primarily on its viscosity (resistance to flow), slope of the ground over which it travels, and the rate of lava eruption.

Because basalt contains the least amount of silica and erupts at the highest temperature compared to the other types of lava, it has the lowest viscosity (the least resistance to flow). Thus, basalt lava moves over the ground easily, even down gentle slopes. Dacite and rhyolite lava, however, tend to pile up around a vent to form short, stubby flows or mound-shaped domes. Components of Igneous Rocks Illustration by J. Johnson When molten rock erupts onto the Earth's surface, it cools quickly, freezing the growth of existing minerals and preventing the development of new minerals. Such rapid cooling will typically produce lava rocks with a few small minerals suspended in a groundmass of volcanic glass. Molten rock that remains below the ground, however, cools very slowly so that existing minerals continue to grow and many new minerals develop. A slow rate of cooling will produce a coarse-grained plutonic rock that consists entirely of large crystals. Different names are given to such slow-cooling plutonic rocks on the basis of chemical composition and mineral proportions (for example, plutonic rocks of basaltic composition are called gabbro).

, This graph shows the volume percent of minerals present in a plutonic rock that consists entirely of crystals. For example, a granite with 70% SiO2 might have 22% quartz, 38% alkali feldspar, 28% plagioclase feldspar, and 12% biotite. The volume percent of minerals present in volcanic rocks typically varies 0-50%. To calculate the relative amounts of the crystals likely to be present, multiply the volume percent in the graph by the actual volume percent in the rock. For example, a rhyolite volcanic rock with 10% crystals, is likely to 2.2% quartz, 3.8% alkali feldspar, 2.8% plagioclase, and 1.2% biotite. (Information courtesy of the USGS)

Question: Of the three types of rock, lava is falls under which type?
Answer: Igneous